Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. The carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. Other thermal spray guns utilize wire as a source of spray material.
Especially high quality coatings of thermal spray materials may be produced by spray guns using oxygen and fuel at very high velocity (HVOF guns). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed into the constricted throat of a short or long gas cap (also sometimes termed nozzle). Powder is fed axially or radially into the combustion chamber or gas cap to be heated and propelled by the combustion effluent to a workpiece being coated.
Examples of HVOF guns are disclosed in U.S. Pat. Nos. 4,417,421 (Browning) and 5,148,986 (Rusch). Generally the powder (or wire) spray material in HVOF guns is introduced internally into a spray passage where there can be a tendency to deposit on the passage walls with resulting buildup. The buildup can dislodge to pass lumps onto the coating, or close down the passage to result in backpressure and attendant malfunction of the gun. U.S. Pat. No. 5,165,705 (Huhne) addresses such deposit by the application of a surface film in the combustion chamber. Reflective surface films have been taught for a different purpose, vis. enhancement of heating, in U.S. Pat. No. 3,055,591 (Shepard). A ceramic flow nozzle is taught in U.S. Pat. No. 5,405,085 (White), wherein the ceramic nozzle absorbs heat from a first portion of flow stream, and transfers the heat to a second portion of the flow stream downstream.
An object of the invention is to provide an improved thermal spray gun, particularly an HVOF gun, having a reduced tendency for buildup in the spray stream passage in the gun. Another object is to provide a novel component for such a gun, such component providing for a reduced tendency for buildup in the spray stream passage in the gun.